(i) Field of the Invention
The present invention relates to: a steel wire for a spring useful as a material for a cold-formed spring used as a suspension spring for an automobile or the like, in particular a steel wire for a spring having both the air-durability and corrosion resistance which are considered to be important characteristics of a spring; and a method useful for producing the steel wire for a spring.
(ii) Description of the Related Art
A cold-formed spring is mainly used as a suspension spring for an automobile and the chemical compositions of steels for springs used as the materials for the springs are stipulated by JIS G3565 to G3567, G4801 and others. In the event of producing a cold-formed spring from such a steel for a spring, a hot-rolled wire rod produced from such a steel for a spring is drawn to a prescribed wire diameter, thus a steel wire is produced and thereafter subjected to oil tempering treatment (austenitizing and tempering treatment), and successively the steel wire is cold-formed into a spring.
A cold-formed spring produced as stated above is required to reduce the size and weight thereof for the reduction of fuel consumption and, as a part of the requirement, a spring of a higher stress is desired and a high-strength steel wire for a spring of 2,000 MPa or more in tensile strength after austenitized and tempered is demanded. In general however, the defect susceptibility of a spring tends to increase as the strength thereof increases and, in the case of a suspension spring used under a corrosive environment in particular, the corrosion fatigue life deteriorates and thus there is fear that breakage occurs at an early stage. It is estimated that the corrosion fatigue life deteriorates because corrosion pits on a surface act as the origins of stress concentration and the generation and propagation of fatigue cracks are accelerated. Therefore, excellent corrosion resistance is a characteristic important for a suspension spring.
In order to adapt to the requirement of higher stress as stated above, various technologies have been studied. As such a means, a method of lowering the tempering temperature at oil tempering treatment (for example, about 400° C.) and thus increasing tensile strength has sometimes been adopted. However, by the method, the toughness and ductility of a steel wire lower, breakage and cracks of the spring material wire are caused during cold-forming, and spring formability is hindered. Further, even when the C content in a steel for a spring is increased and thus tensile strength is increased, still not only toughness and ductility deteriorate and thus spring formability is hindered but also corrosion resistance deteriorates, and the quality as a suspension spring for an automobile cannot be secured.
In contrast, a means of adding great amounts of alloying elements such as Ni, Cu, Cr, Si, etc. and thus improving corrosion resistance is also considered. However, when such a means is adopted, not only the cost of a steel material increases, but also the hardenability of the steel increases, thereby the proportion of martensite and bainite structures in the structure after hot-rolling increases, resultantly toughness and ductility are lowered, and drawbacks such as breakage of the wire during subsequent cold-drawing are caused.
As stated above, to realize a steel wire having both the characteristics of high tensile strength and good corrosion resistance is in a difficult situation at the present day. Then, various technologies have been proposed in order to improve such problems. For example, U.S. Pat. Nos. 5,508,002 and 5,846,344 propose a means of: controlling the combination of components so that an FP value stipulated by the following expression (5) may be in the range from 2.5 to 4.5; thereby inhibiting martensite and bainite structures after hot-rolling; and resultantly inhibiting the deterioration of formability caused by the addition of alloying elements. Such a means is a technology which: is based on the addition of alloying elements which improve corrosion resistance; and further improves the corrosion resistance by reforming the austenitized and tempered structure. However, the improvement of corrosion resistance by the technology is limited.FP=(0.23[C]+0.1)×(0.7[Si]+1)×(3.5[Mn]+1)×(2.2[Cr]+1)×(0.4[Ni]+1)×(3[Mo]+1)  (5),where [C], [Si], [Mn], [Cr], [Ni] and [Mo] represent the contents (mass %) of C, Si, Mn, Cr, Ni and Mo, respectively.
Further, Japanese Patent No. 3429258 discloses a means of attaining both high tensile strength and good corrosion resistance by controlling the content of Cr to 0.25% or less and further controlling the contents of Cr, Cu and Ni so as to satisfy the relationship stipulated by the following expression (6). However, even by the technology, steel material component design has to be carried out within a regulated range of chemical component compositions and thus the improvement of corrosion resistance is limited.[Cr]≦([Cu]+[Ni])/2  (6),where [Cr], [Cu] and [Ni] represent the contents (mass %) of Cr, Cu and Ni, respectively.
Furthermore, U.S. Pat. No. 6,338,763 proposes a technology of improving formability by controlling the amount of retained austenite (retained γ) to 6 vol. % or less and thus reducing the induced transformation of the retained austenite during the cold-forming of a spring. However, the technology is basically aimed at the improvement of formability and does not take the improvement of corrosion resistance into consideration at all.
In the meantime, it is known that a method of fining austenite grains is also useful as a means for inhibiting the deterioration of toughness, ductility and resistance to hydrogen embrittlement accompanying the increase of the strength of a steel for a spring. As such a means for example, U.S. Pat. No. 5,776,267 discloses a method for fractionizing the sizes and structures of carbide and nitride in order to improve resistance to hydrogen embrittlement. However, even when the technology is adopted, the size of austenite grains is limited to No. 11 in austenite grain size number and the improvement of corrosion resistance is limited accordingly.